1. Field of the Invention
The present invention relates to the field of optical recording and reproduction of information on optical disks and similar optical information storage media. More particularly, the invention relates to an improved shock-resistant pick-up apparatus for directing radiant energy onto precisely controllable locations of an optically sensitive recording medium to record information on the medium, and/or to reproduce information previously recorded on the medium.
2. Description of Background Art
Optical storage of information on video disks has come into widespread use for a variety of applications. Video disks using pulse width modulated (PWM) and pulse code modulated (PCM) signals containing audio, video, alphanumeric information, as well as binary encoded data in general, are in common use. Video disk systems use an electro-optical device, usually called a pick-up, to focus on and track information-containing regions of the disk, in which information bits are recorded by altering an optical property of the disk. Individual bits of bi-directional information may be recorded in spaces having a maximum dimension of less than 1 micron. Typically, these regions are arranged in the form of a continuous spiral track, or a series of concentric tracks, containing a string of bits. Precise tracking is required to select a track, or to maintain a selected track, or portion of a track, within the optical field of view of the pick-up. Precise focusing is also required to recover closely packed information bits contained in a track.
Focusing and tracking of information bits are usually performed by a servomechanism which moves the objective lens of an optical pick-up apparatus. Normal motion of the pick-up with respect to the information-bearing surface of the rotating disk performs focusing, while radial motion of the pick-up performs tracking. Motion required for focusing on the information bearing surface of the video disk typically is achieved by enclosing the lens in a relatively co-planar ring. The ring is wound with an electrical coil, which in turn is mounted flexibly within a concentric circular magnet. The coil and magnet combination resembles the voice coil and magnet arrangement found in common audio loudspeakers. Optical energy focused by the lens onto a photodetector permits closed-loop servo control of the relative distance between lens and disk.
Tracking of information tracks on a rotating video disk is typically accomplished by mounting one or more electrical tracking coils onto the voice coil ring, radially distant from but normal to the longitudinal axis of the pick-up. When electrically energized, the tracking coils are attracted angularly about the axis of the pick-up relative to poles provided within the concentric permanent magnet enclosing the voice coil. Now, the optical axis of the objective lens is fixed eccentrically from and parallel to the longitudinal axis of the pick-up. Therefore, when the voice coil ring is rotated bi-directionally about the pick-up axis, the optical axis of the lens moves bi-directionally in a radial arc lying in a plane parallel to the information-bearing surface of the disk. For small arc lengths, this bi-directional motion is substantially along a radius of the disk.
Usually, a movable mechanical support device or transport is used to position the pick-up at a desired approximate position relative to a disk. The pick-up is held and moved radially across the rotating video disk during play by a servo-mechanism that controls the transport which, for example, may be of the radial guided rail type or the swing arm type, both derived from the phonograph record player art.
Phonograph record players are sensitive to external disturbances and often skip grooves as a result of such disturbances. Similarly, video disk players may respond to shakes and bumping by skipping turns of an optical information track, and/or by defocusing. Track skipping and defocusing in video disk players occur because the dynamic tracking range of existing electro-optical pick-ups is limited by existing designs to a relatively small number of turns of the optical track. Although the transport mechanism is controlled by a servo mechanism to maintain the position of the pick-up near the center of its dynamic range, the inertia of the pick-up and the transport in combination, during the presence of shaking or bumping, may require tracking maintenance forces beyond the system capabilities.
Several technologies are currently used to minimize the effect of video disk track skipping resulting from shock or vibration. Loss of focusing under such conditions is not compensated for in most currently existing systems. In video disk systems adapted to accommodate disks of various diameters and designed for portability, installation in automobiles, and the like, shock mounting or cushioning is used to minimize the effects of shock and vibration. However, such cushioning may not adequately isolate the systems from unusually large accelerations. Video disk systems used in stationary installations, such as studios or homes, sometimes employ a combination of massive construction and cushioned mounting of the apparatus in order to reduce the effects of minor shaking.
Another system for reducing optical track-skipping during accelerations caused by shock or vibration utilizes electronic circuitry to delay and continuously store the information stream from a limited number of turns of the disk in an electronic memory device, of the clocked First-In-First-Out (FIFO) type. During accelerations which cause skipping, information is outputted without interruption from the FIFO at the clock rate while the pick-up is restored to the point in that optical track where skipping began. The period during which this restoration occurs is compensated for by a temporary acceleration in disk rotational speed.
Taking into account the relatively heavy masses of the permanent magnet, focusing and tracking windings, objective lens and related elements of a typical pick-up, and its rigid transport mechanism in the video disk player, efforts are being made to reduce the masses of the pick-up and transport, thereby achieving some reduction in susceptibility to vibration, bumping and shocks. Integrating the optics and laser within a single solid state device of relatively small size is one approach being used to achieve the desired mass reduction. This approach is described in "Integrated-Optic Disc Pick-Ups," Nishihara et al., Optical Data Storage, v. 1663, p. 26, SPIE-Intl. Soc. Opt. Engrg., 1992.
In video disk player apparatus, the locational tolerances relating the axis of the turntable motor, the pick-up transporting means, and the optical axis of the pick-up, all require a high degree of precision in the manufacture of the component parts and in the assembly and test of the finished product. Means are provided by my invention to loosen such locational tolerances, reduce the need for precision, and simplify the assembly and test of video disk apparatus.
The present inventor is aware of the following U.S. Patents which may have possible relevance to the present invention:
______________________________________ U.S. Pat. No. Inventor Issue Date ______________________________________ 2,113,184 Sperti 04/04/38 3,530,258 Gregg 09/22/68 4,092,529 Aihara, et al. 05/30/78 4,252,412 Camerik 02/24/81 4,269,486 Shintani 05/26/81 4,302,830 Hamaoka, et al. 12/24/81 4,322,837 Mickleson, et al. 03/30/82 4,337,531 Willemsen 06/29/82 4,342,935 Kallmeyer, et al. 08/03/82 4,367,543 Araki, et al. 01/04/83 4,488,789 Kenney 12/18/84 4,672,594 Kato, et al. 06/09/87 4,794,581 Andresen 12/27/88 4,807,213 Chung, et al. 02/21/89 4,822,139 Yoshizumi 04/18/89 4,857,719 Ando 08/15/89 4,927,235 Narumi 05/22/90 4,939,711 Yoda 07/03/90 4,942,562 Suzuki 07/17/90 4,945,526 Jansen, et al. 07/31/90 4,945,527 Sunagawa 07/31/90 4,945,529 Ono, et al. 07/31/90 4,948,230 Kasahara, et al. 08/14/90 ______________________________________
All relevant prior art that the present inventor is aware of disclose the use of conventional electromagnetic or piezoelectric devices to effect focusing and/or tracking of an optical beam on an optical recording medium. Devices of this type have inherent drawbacks such as limited frequency response, limited deflection range, excessive weight or cross-axis coupling.
None of the prior art which the present inventor is aware of provides a completely satisfactory solution to the problem of accurately controlling the tracking and focus of pick-up heads used in optical recording. The present invention was conceived to provide an improved pick-up apparatus for optical recording that employs electrostatic forces and overcomes certain limitations of prior art pick-up devices.